Modern large sea going vessels are built using a modular system. Modern aircraft carriers, super-liners and the mega-tankers / container-haulers are all built in a similar manner. Individual modules, or "lifts" are constructed and integrated together as completed. This provides a very flexible and much more manageable construction project in the long term.

It wouldn't be beyond the realm of consideration that ships such as the Galaxy or the Odyssey classes could be constructed using similar techniques. The individual modules could be constructed on a planet's surface and "lifted" to the final assembly facility in orbit much more economically than fully completed large sections such as saucer sections and engineering hulls. Mars' weaker gravity and thinner atmosphere would also offer certain advantages to a ground-construction technique as modules could be constructed larger than at a higher gravity / thicker atmosphere facility. Also consider that this is the same technique used to build the International Space Station, thus we already have the underpinnings for constructing large spacecraft in such a way.

If you can accept the various Technical Manuals, Starfleet vessels are very much modular. The Warp Nacelles are designed to be modular and easily swappable (in a ship yard). The ships computer core could be built planetside and the transported into orbit for installation.

On the Galaxy Class, ship yards massive transporters to transport various pre-built module rooms to outfit the ship. They could put in various labs or quarters with specialized life support equipment to support non M-Class lifeforms. (The technical mauals mention that upto 35% of the Enterprises's primary hull wasn't in used. )The bridge of the Starship is an entire module onto itself. Bridges can be swapped in and out of ships at a ship yard (that explains different bridges being shown on different starship of the same class). The bridge of the Intrepid Class (e.g. Voyager's Class) could be jettisoned from the ship in an emergency and used as a massive life capsule.

Building large stations or ships in space is the only practical step to advancing our race. You know it requires like 1000 pounds of thrust per pound on the space shuttle. That is why building a space elevator would make expanding the ISS financially possible.

Bringing supplies in bulk via space elevator will enable a so called "space boom" that will enable us to transport supplies and materials so quickly and cheaply (compared to the space shuttle) we can construct so much more! Plus in case of an emergency personnel can evacuate to the elevator.

The Japanese are said to be constructing the first space elevator, the most important part are the cables. At this point they say they're about 10 years away from making strong enough cables. Progression of technology and such.

The cables for the space elevator are supposed to be constructed of Carbon Nanotubes

"A multi-walled carbon nanotube was tested to have a tensile strength of 63 gigapascals (GPa). This translates into the ability to endure tension of a weight equivalent to 6422 kg (14,158.09 lb) on a cable with cross-section of 1 square mm (0.03937 square in)"

This was possible in 2000. They have improved this quite a bit since then.

In a future where the ability to go many times the speed of light is possible, not to mention the ability to utilize transporter technology that has already been proven to be impossible, its conceivable that carting large modules into space is more economical than it is to us now.

Modular construction is far more efficient, not to mention building a massive ship of that sort of complexity entirely in vacuum would be incredibly slow. EVAs in the Star Trek future dont' seem to be that much more dexterous so doing the kind of fine manual work necessary to build a ship like The Enterprise etc would be... extremely slow.

Construction within the atmosphere is really the only viable way to do it. We can assume that those crafty Starfleet engineers could find a way to lift it into orbit piece of by piece I'm sure.

In a future where the ability to go many times the speed of light is possible, not to mention the ability to utilize transporter technology that has already been proven to be impossible, its conceivable that carting large modules into space is more economical than it is to us now.

On the point: "why is there no Enterprise class": With regard to the fact that Starfleet sees the name "Enterprise" as an epic ship, They would not build a ship, call it Enterprise, take it through shakedown and then say: Oh boy... The Enterprise class is a epic failiure... let's scrap this model.

In a future where the ability to go many times the speed of light is possible, not to mention the ability to utilize transporter technology that has already been proven to be impossible, its conceivable that carting large modules into space is more economical than it is to us now.

Modular construction is far more efficient, not to mention building a massive ship of that sort of complexity entirely in vacuum would be incredibly slow. EVAs in the Star Trek future dont' seem to be that much more dexterous so doing the kind of fine manual work necessary to build a ship like The Enterprise etc would be... extremely slow.

Construction within the atmosphere is really the only viable way to do it. We can assume that those crafty Starfleet engineers could find a way to lift it into orbit piece of by piece I'm sure.

What I find funny about that argument is that people said the same thing for breaking the sound barrier and space flight in general when it first started. It couldn't be done, it would kill who ever tried it yadda yadda...
Yet we found ways to do both. And now look back on it and ask what the big deal was.

Building part of the ship on the ground makes sense. It would be much easier for the workers to assemble and construct certain parts with out having to wear a EV suit, or work in a workerbee. Then when the parts are done they are just lifted into orbit (with big tractor beams or something) and final assembly is completed there. Submarines spend their entire life in the water, yet we don't build them there. They are built where it is easiest for the workers to put it together, then it's put in the water.